My understanding is that 2019.12 was compatible with 2.5 an 3. Then 2019.16 came out originally for 2.5 only. Then 2019.16.3 came out specifically for HW3. HW3 also stores video files in a newer format. If you can park in an area with good Wi-Fi, 16.3 seems to be rolling out pretty quickly now for HW3.
I have not received my M3 yet, so I cannot verify any of this.
My current theory is that Tesla simply ran out of HW3 computers, and started using old parts to meet production. I sincerely hope this theory is proven to be wrong.
HW2.5 even after April 12th build date